First Principles Study of Ferroelastic Twins in Halide Perovskites

We present an ab initio simulation of 90° ferroelastic twins that were recently observed in methyl ammonium lead iodide. There are two inequivalent types of 90° walls that we calculate to act as either electron or hole sinks which leads us to propose a mechanism for enhancing charge carrier separation in photovoltaic devices. Despite separating non-polar domains, we show these walls to have a substantial in-plane polarization of ∽ 6 μC cm-2, due in part to flexoelectricity. We suggest this in turn could allow for the photoferroic effect and create efficient pathways for photocurrents within the wall.